ARM I2_S inference produces gibberish/garbage output after commit 112f853 (CPU Optimization update) #470

Description

After the CPU Inference Optimization update (commit 112f853), running inference with i2_s quantization on ARM (aarch64) produces completely incoherent output — random tokens with no relation to the prompt. Rolling back to commit 404980e (the last commit before the optimization merge) restores correct, coherent output.

Environment

• Hardware: Raspberry Pi 5 (8GB RAM), ARM Cortex-A76 (aarch64)
• OS: Raspberry Pi OS 64-bit (Debian 12 Bookworm)
• Compiler: Debian clang version 18.1.8
• CMake: 3.25.1
• Python: 3.9 (conda)
• Model: microsoft/BitNet-b1.58-2B-4T-gguf (ggml-model-i2_s.gguf)
• Quantization: i2_s

Steps to Reproduce

1. Clone repo at current HEAD (01eb415):

   git clone --recursive https://github.com/microsoft/BitNet.git
   cd BitNet
   

2. Generate kernels and build (following Adafruit guide):

   python utils/codegen_tl1.py --model bitnet_b1_58-3B --BM 160,320,320 --BK 64,128,64 --bm 32,64,32
   export CC=clang-18 CXX=clang++-18
   rm -rf build && mkdir build && cd build
   cmake .. -DCMAKE_BUILD_TYPE=Release
   make -j$(nproc)
   cd ..
   

3. Download model:

   huggingface-cli download microsoft/BitNet-b1.58-2B-4T-gguf --local-dir models/BitNet-b1.58-2B-4T
   

4. Run inference:

   python run_inference.py -m models/BitNet-b1.58-2B-4T/ggml-model-i2_s.gguf -p "You are a helpful assistant" -t 4 -cnv
   

Broken Output (HEAD - 01eb415)

> hi how are you
ri differentorefFly increase Hurtutar run following section underestimateAD Sachs weighedision
cann RICTS Reyn taskfir-ra mark filtr castWATCHB fr ret flatten missionuche purchase parameter
gramhit associatedyuraft runeded take compound sugar contrast unsubedom conveyuffanford...

Working Output (commit 404980e)

> hi
Hello! How can I assist you today?

> what is a raspi 5
The Raspberry Pi 5 is a next-generation model of the Raspberry Pi single-board computer series...

Performance on the working commit: 9.68 tokens/second (4 threads, ARM NEON).

Bisection

The regression was introduced in commit 112f853:

112f853 [feat] I2S kernels for weight & activation parallel on Intel & ARM machine;
        [feat] I2S GEMV & GEMM(llama.cpp);
        [feat] quantize activation & dequantize embedding(llama.cpp);
        [fix] compile bug: cannot define __ARM_FEATURE_DOTPROD(llama.cpp)

The last known working commit is 404980e (one commit before 112f853).

Notes

• The build completes without errors on both commits — the issue is runtime behavior, not compilation.
• ggml-bitnet-mad.cpp is compiled and linked in both cases.
• NEON is detected and enabled (NEON = 1 in system_info output).
• DOTPROD detection: GGML_COMPILER_SUPPORT_DOTPROD - Failed, but COMPILER_SUPPORTS_ARMV82_DOTPROD - Success.
• This issue also appears to affect other ARM64 platforms (Ampere/Hetzner CAX servers), not just Raspberry Pi.
• The Adafruit BitNet on Raspberry Pi guide (published Sept 2025, before the optimization commit) confirms working output on Pi 4 and Pi 5 with the older codebase.

Related to #411 — same root cause. Adding Pi 5 (Cortex-A76 with dotprod) as another confirmed affected platform.